Film Bulk Acoustic Resonator (commonly known as FBAR) are used in a variety of applications, for example as filters in wireless communication systems. When used in wireless communication, single membrane FBAR filters have been used primarily on the receiving side because the receiving side usually involves substantially lower power levels. Single-membrane FBARs have not been used extensively on the transmission side because the high power levels involved generate a lot of heat in the FBARs, but the construction of the FBARs prevents sufficiently rapid transfer of the thermal energy.
FIGS. 1A and 1B illustrate a film bulk acoustic resonator (FBAR) array 100. FIG. 1A illustrates that the FBAR array 100 comprises a piezoelectric membrane 102 having a plurality of individual FBARs 104 arranged thereon. The individual FBARs 104 are electrically connected to at least one other FBAR by interconnects 106 through either top or bottom electrodes. FIG. 1B illustrates a cross section of the FBAR array 100. The piezoelectric membrane 102 is suspended along at least two of its edges by supports 112, and the array includes a plurality of individual FBARs 104, each comprising a portion of the membrane 102 sandwiched between a first electrode (in this instance the upper electrode 108) and a second electrode (in this instance the lower electrode 110). The active area of each FBAR is the portion of the piezoelectric membrane in which the first and second electrodes overlap, because only the area where each FBAR's first and second electrodes overlap—in other words, the area between electrodes—can be subject to an applied electric field.
During operation of the FBAR array, a signal is input to each of the FBARs 104. As a result, heat is generated in the piezoelectric area and the active area experiences a temperature rise. The only means by which the thermal energy can be transferred away from the active areas is laterally through the membrane, as illustrated by the arrows in the figure. The thermal energy travels through the membrane 102 and is dissipated into the supports 112. Since the center of the membrane is farthest from the supports 112, the thermal energy generated by an FBAR at or near the center of the membrane dissipates more slowly and that area experiences a greater temperature increase. For FBARs closer to the edge of the membrane (and thus closer to the supports) the heat dissipates more quickly and the temperature increase experienced by these FBARs is substantially lower. In applications in which a substantial amount of power is input to the FBAR array (e.g., transmission applications in which the power input can exceed 1 W), the temperature rise at the center of the FBAR array can exceed 100 degrees. Such large temperature rises can shift the resonant frequency of the FBAR out of specification, and in some cases can damage the FBAR array and render the entire thing useless.